Real-time spectral response guided smart femtosecond laser bone drilling

Bone drilling is a common surgical procedure of knee arthroplasty, craniotomy, stapedotomy, dental treatment, and laminotomy. Although femtosecond laser processing provides a promising method for precision bone drilling, it is still challenging to achieve thermal accumulation suppression and damage protection for large-scale hole drilling. In this paper, we propose a real-time spectral response guided femtosecond laser bone drilling method with the second-harmonic-generation (SHG) signals for positioning and monitoring. SHG green signal centered at 513.5 nm is generated below laser ablation threshold of bone, which can provide as a label-free detection for initial positioning. Results show that the spectral intensity ratio of green and white signal gradually decreases due to the accelerated removal of hydroxyapatite with the increased laser fluence, while the spectral intensity ratio of red signal and other visible signal can be used to indicate the thermal damage due to the abnormal enhancement of the red signal ranging from 700 to 800 nm when carbonization occurs. Based on the real-time spectral response at different laser fluence, the key laser processing parameters including defocus distance, flow rate of cooling water, and laser fluence are successfully optimized. Finally, a damage-free hole with the average diameter of 3 mm and the depth of 3.6 mm is successfully drilled within 5 min, while the temperature of surrounding hole wall is 37.1 °C. The proposed method provides a direct-viewing, damage-free and high-efficiency way of laser bone drilling with real-time positioning and monitoring.